A. Field of Invention
The present invention relates to the field of navigation and, more particularly, to a method and system for processing pulse signals within an inertial navigation system.
B. Description of Related Art
An inertial navigation system typically uses inertial sensors to measure rotational and linear movements without reference to external coordinates. Systems, such as an Inertial Navigation System (INS) and/or an Inertial Measurement Unit (IMU) typically use inertial sensors, such as accelerometers and gyroscopes. An INS is a self-contained box, which provides outputs indicative of position, velocity, attitude, and heading. An INS typically has three accelerometers, and to obtain desired outputs, it is typically necessary to maintain the accelerometers pointing in a north, east, down reference frame (down is a local direction obtained with reference to gravity). The accelerometers within an INS output a signal that is a function of acceleration. Integrating the outputs of the accelerometers produces velocity outputs and integrating again produces position outputs.
Unfortunately, the accelerometers typically have imperfections, resulting in random output errors. Some of the errors are associated with an unstable arrangement of the accelerometers. To maintain the accelerometers in the north, east, and down (or similar orthogonal) reference frame, they may be placed in a gimbaled assembly, and using gyroscopes, the gimbaled assembly may be maintained in this north-east-down frame. However, like the accelerometers, the gyroscopes are typically imperfect, which also results in random errors. In addition, using the gimbaled assembly creates positioning and manufacturing difficulties.
To solve the problems associated with using the gimbaled assembly, the inertial sensors can be securely mounted in place. This results in a “strap-down” system, such as a strap-down INS or a strap-down IMU, which has inertial sensors rigidly fixed (i.e., strapped down) to the chassis of a moving body. Therefore, strap-down systems move with the moving body. Sensors of strap-down systems experience and measure changes in reference to the body's fixed axes instead of the north-east-down reference frame. The body's fixed axes are a moving frame of reference as opposed to a constant inertial frame of reference provided with the gimbaled assembly.
In order to interpret sensor outputs of the strap-down system related to the north-east-down reference frame rather than to the moving body's reference frame, the sensor outputs are transformed mathematically from the moving body's coordinate system to a three-dimensional reference coordinate system. The mathematical transformation allows the strap-down system to function as a gimbaled INS would.
The mathematical transformation is done in a digital computer within the strap-down system, and therefore the strap-down system has limits as to how fast it can repetitively calculate the new reference frames. The computer within the strap-down system may also compensate the sensor outputs to correct for processing errors. The computer accumulates a large group of sensor outputs and then processes the groups of outputs together in order to establish a new reference frame and to correct for processing errors. This results in slow processing due to grouping of outputs. The slow processing causes errors such as coning and sculling to occur due to the slow integration of the inertial signals. For example, the computer will receive outputs from inertial sensors simultaneously and on a continuous basis. Since the computer accumulates outputs prior to processing, the computer will not know the sequence of the outputs that were produced because the computer simply receives a number of signals and processes the signals together as a group. As such, navigation errors will accumulate within the calculated reference frame outputs. Thus, digital computer processing and software based compensation of groups of sensor outputs may not be a practical method of processing inertial sensor outputs for high dynamic conditions.